Persistent viral infections, including HIV, hepatitis B and C are major causes of morbidity and mortality worldwide. Despite our successes with acute infections using anti-microbial agents and vaccines, persisting viral infections cause considerable disease. CD8 T cells responding to chronic viral infections become dysfunctional, a state termed exhaustion. Transcriptional profiling of mRNA has revealed major insights into the biology of T cell exhaustion including the identification of key transcription factors, as wellas immunoregulatory pathways, such as PD-1 and LAG-3 that have major roles in controlling T cell exhaustion. These latter findings have led to highly promising clinical trials for treating chronic infectious diseases and cancer. Despite these advances, the molecular mechanisms of CD8 T cell exhaustion remain incompletely defined. Specifically, our understanding of the gene expression program of T cell exhaustion has been limited so far to only protein-coding mRNA. In the past decade, non-coding RNAs and especially microRNAs (miRs) have been identified as crucial regulators of cell development, differentiation, and function. However, the role of miRs in immunity to infection is only beginning to be revealed and specifically the role of miRs in regulating CD8 T cell exhaustion during chronic viral infection is unknown. Our preliminary data indicate that miR-155 and other miRs are associated with differences between memory and exhausted CD8 T cells, but precisely how these non-coding RNAs regulate T cell exhaustion is unclear. Thus, we hypothesize that specific miRs, including miR-155, regulate the differences in differentiation, function and transcription of protein coding genes between functional memory and exhausted CD8 T cells. To test this hypothesis we propose: Aim 1. To investigate the role of miR-155 in chronic viral infection. We will manipulate miR-155 expression to test how this miR regulates the development and/or persistence of exhausted CD8 T cells during chronic viral infection. Aim 2. To identify novel miRs regulating CD8 T cell exhaustion during chronic viral infections. We will test the role of novel differentially expressed miRs in regulating exhaustion v memory. Clinical approaches to modulate T cell exhaustion based on targeting PD-1 and other inhibitory receptors are highly promising for the treatment of cancer and chronic infections. However, with efficacy ranging from 10- 50%, other approaches and a deeper understanding of T cell exhaustion is clearly warranted. With clinical approaches to modulate miRs rapidly developing, the proposed studies will provide a foundation for future miR-based therapeutic advances in infection and cancer.